A review of solar collectors using carbon-based nanofluids

Journal of Cleaner Production 241 (2019) 118311

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Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro

Review

A review of solar collectors using carbon-based nanofluids Adeola Borode a, *, Noor Ahmed a, Peter Olubambi b a b

Department of Mechanical Engineering Science, University of Johannesburg, Johannesburg, South Africa Department of Metallurgy, University of Johannesburg, Johannesburg, South Africa

a r t i c l e i n f o

a b s t r a c t s

Article history: Received 8 July 2019 Received in revised form 20 August 2019 Accepted 5 September 2019 Available online 6 September 2019

The quest to enhance the thermal efficiency of solar collectors by improving the working or absorbing fluid led to the synthesis of nanofluids. Numerous studies have highlighted different nanomaterials such as copper oxide, alumina, silica and so on for dispersion in working fluid and subsequent application in solar collectors. However, carbon nanomaterials have been adjudged as the most promising for preparing nanofluids and heat transfer application. This is because carbon nanomaterials possess remarkable thermophysical properties. These properties contribute a notable enhancement in the thermophysical properties of the working fluid and consequently improve the performance of solar collectors. This study succinctly presents an overview of the performance of various solar collectors utilizing carbon-based nanofluids. The influence of nanofluid concentration, temperature and flow rate on the collector efficiency of the solar collectors were highlighted. This research outcome showed that carbon-based nanofluids with a low concentration of about 0.3 vol% improved the collector efficiency of flat-plate, evacuated-tube, parabolic trough and hybrid photovoltaic thermal solar collector up to 95.12%, 93.43%, 74.7% and 97.3% respectively. Also, direct absorption solar collector using carbon-based nanofluid with a low concentration of at least 0.01 vol% achieved a photothermal efficiency of up to 122.7%. The study further revealed that there is a huge potential to achieve the application of carbon-based nanofluid on a commercial scale. The challenges and prospects for further research were identified. © 2019 Elsevier Ltd. All rights reserved.

Handling editor: Sandro Nizetic Keywords: Graphene Carbon nanotubes Nanofluids Solar collectors Heat transfer Performance

Contents 1. 2. 3.

4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1. Thermophysical properties of carbon-based nanofluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Thermal application of carbon-based nanofluids in solar collectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.1. Flat plate solar collector (FPSC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.2. Evacuated tube solar collector (ETSC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.3. Direct absorption solar collector (DASC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.4. Parabolic trough solar collector (PTSC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3.5. Hybrid photovoltaic thermal (PVT) collector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

1. Introduction

* Corresponding author. E-mail address: [email protected] (A. Borode). https://doi.org/10.1016/j.jclepro.2019.118311 0959-6526/© 2019 Elsevier Ltd. All rights reserved.

At present, the global community is confronted with energy crisis due to the gradual increase in the world's population and its dramatic demands for energy in transportation, electricity, heating

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A. Borode et al. / Journal of Cleaner Production 241 (2019) 118311

Nomenclature

Symbols Ag Al2O3 CO2 Cu CuO hconv H L _ m Q Re SiO2 SO2 T0 T∞ ╤out TiO2

Silver Aluminium Oxide; Carbon dioxide; Copper Copper Oxide; convection heat transfer coefficient (W/m2.K) depth of collector (m) length of collector (m) mass flow rate per unit width (kg/m-s) heat generation in the collector (W/m3) Reynolds number, rVL=m Silicon dioxide; Sulphur dioxide; inlet temperature (K) ambient temperature (K) average temperature at the outlet (K) Titanium dioxide; V, Velocity (m/s)

Abbreviations CNTs Carbon Nanotubes CTAB Cetyltrimethylammonium Bromide; DASC Direct Absorption Solar Collector

process etc. Also, global energy demand is expected to rise by one third in 2040 (BP, 2018). This increasing energy demand has placed a strain on fossil fuels (such as crude oil, coals, natural gas etc.) due to overconsumption. Thus, there is a risk of depletion or a possible shortage of fossil fuels in decades to come (Farhad et al., 2008). Moreover, the enormous use of these fossil fuels negatively affects the environment through air pollution, global warming and climate change (Crutzen et al., 2008; Perera, 2017). Therefore, due to the finite reserve of fossil fuels and their environmental impacts, transition to clean renewable and sustainable energy sources is seen as the way to reduce the global energy crisis and dependence on fossil fuels (National Academy of Sciences, 2010; Panwar et al.). This is because the supply of energy from renewable sources is infinite. Despite constituting the highest increase in global power generation in 2017, renewables currently account for a little above 27% of the total global power while fossil fuel accounts for a total of over 70% inclusive of coal, which is about 38% (World energy outlook, 2018). This shows there is still a long way to go to completely transition from fossil fuel to renewable. However, renewable energy consumption has been on the rise over the last decade as depicted in Fig. 1. By 2040, renewables are expected to substantially supply 40% of the global power mix, 25% of heat demand and 19% of transportation energy demand (World energy outlook, 2018). In addition, renewables were predicted to contribute 41% of the total global reduction in CO2 emission while energy efficiency is expected to contribute 40% (Renewable Energy Agency, 2018). Renewable energy source includes solar energy, hydropower, wind energy and biomass energy. Of all these renewables, solar energy is seen as the most abundant and have the least impact on the environment (Lee et al., 2018). However, it only constituted the lowest percentage of renewable energy consumption (BP, 2018). This could be attributed to the high cost of setting up a solar conversion system and its very low efficiency. Due to positive government policies around the world, solar usage is expected to grow

DIW DW EG ETSC FPSC GA GO GONP GNP HTC MLG MWCNT NDG PTSC PV PVT PG SDBS SDS SWCNT vol% wt%

De-Ionized Water Distilled Water Ethylene Glycol Evacuated Tube Solar Collector Flat Plate Solar Collector Gum Arabic Graphite Oxide; Graphene Oxide Nanoplatelet; Graphene Nanoplatelet; Heat Transfer Co-efficient Multi-Layered Graphene Multi-Walled Carbon Nanotubes Nitrogen-Doped Graphene Parabolic Trough Solar Collector Photo-Voltaic Photo-Voltaic Thermal Polyethylene Glycol Sodium Dodecylbenzene Sulfonate Sodium Dodecyl Sulfate Single-Walled Carbon Nanotubes Volume fraction of nanomaterials Weight fraction of nanomaterials

Greek Symbols Р Density (Kg/m3) М Dynamic Viscosity (Kg/m.s)

substantially by 2050. Fig. 2 shows the forecasted percentage of different sources of renewable energy and its applications. Solar photovoltaic (PV) and thermal are the major systems used to take advantage of the solar energy to generate electricity and heat space, liquid or any other medium (El Bassam et al., 2013). The solar thermal system application for heating is on the rise (Martinopoulos, 2016). The solar thermal system includes concentration and non-concentrating system (Kalogirou and

Fig. 1. Growth trend in Renewable energy 2010e2018 (International Renewable Energy Agency, 2018 (IRENA)).

A. Borode et al. / Journal of Cleaner Production 241 (2019) 118311

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Fig. 2. Renewables source and applications (Renewable Energy Agency, 2018).

Kalogirou, 2009a). Concentrating systems such as concentrating solar panels are used to generate electricity while nonconcentrating systems are used to domestically heat space and water or for process heating in industries (El Bassam et al., 2013). To achieve the thermal process, solar collectors are used. A solar collector is a heat exchanging device used to convert solar energy absorbed from incident solar radiation to thermal energy (Tripanagnostopoulos, 2012). The different types of solar collectors include flat-plate solar collectors, evacuated tube solar collectors, direct absorption solar collectors, parabolic trough solar collectors and hybrid photovoltaic thermal collectors (Kalogirou, 2016; Tripanagnostopoulos, 2012). Fig. 3 presents the different types of solar collector and applications. To improve the efficiency of these solar collectors, nanofluids were incorporated as the absorber fluid in place of the traditional fluid such as water, ethylene glycol (EG) etc. Nanofluids are prepared by suspending nanomaterials (such as metal oxides, ceramics, carbon nanostructured materials) into base fluids (such as water, EG, oil etc.) (Haddad et al., 2014; Yu and Xie, 2012). The preparation method is either by one-step method or two-step method. One-step method involves the synthesis of nanomaterials and its simultaneous dispersion in base fluids while two-step method is the synthesis of dry nanomaterial powder and subsequent dispersion in base fluid (Devendiran and Amirtham, 2016; Mahbubul and Mahbubul, 2019). The latter is the most widely used and most economical of the two but results in poor nanofluid stability. This is ascribed to the high aspect ratio and hydrophobic properties of the nanomaterials. This instability is the tendency of the nanomaterials to agglomerate. This can result in blockage of flow channels. However, the stability can be improved either by acid/alkaline covalent functionalization of nanomaterials (Sadri et al., 2018; Shazali et al., 2018b) or the more economical non-covalent functionalization with the aid of surfactants or polymers such as SDS (Sarsam et al., 2016), SDBS (Mahmudul

Haque et al., 2015), GA (Cheng et al., 2014), Tween 80 (Askari et al., 2016), chitosan (Phuoc et al., 2011), Pluronic P-123 (Shazali et al., 2018a), etc. In contrast to the two-step method, the onestep method is mostly ignored in a lot of studies because it involves the use of costly equipment coupled with its complex production processes (Babar et al., 2019). Numerous researchers have investigated the performance of solar collectors using nanofluids prepared with different nanomaterials, which include CuO, Al2O3 (Khan et al., 2019; Pise et al., 2016), MgO (Dehaj and Mohiabadi, 2019), CeO2 (Sharafeldin and  f, 2018) etc. Also, most of the review studies available focused Gro more on Al2O3 nanofluid than other types of nanofluids. However, there are little to no review that focuses mainly on the performance of carbon-based nanofluids in different solar collectors. In this regard, this study focused mainly on the application of carbon-based nanofluid in solar collectors. This is because carbon nanomaterials such as graphene and CNT possess higher thermal conductivity when compared to other nanomaterials such as Al, Al2O3, Cu, CuO etc., which are commonly used for preparing nanofluids (Balandin, 2011). Due to the exceptional thermal conductivity of carbon nanomaterials, their nanofluids possess a higher thermal conductivity and heat transfer coefficient than conventional base fluids and other nanofluids of metal and metal oxides. Also, carbon nanomaterials exhibit a lesser density, larger surface area, elevated stability with minimal erosion and corrosion than other nanomaterials (Murshed and Nieto de Castro, 2014; Sadeghinezhad  et al., 2016). It is also worthy to note that graphene (Sest et al., €m, 2016) are excellent anticor2018) and CNTs (Chen and Bostro rosion additives or protective coatings for spectrally selective solar absorbers. These remarkable properties indicate that carbon nanomaterials are ideal nanomaterials that should be utilized for the production of nanofluids for heat transfer application. Thus, it is pertinent to thoroughly and meaningfully assess the research trends, achievement and development in the enhancement of the

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A. Borode et al. / Journal of Cleaner Production 241 (2019) 118311

Fig. 3. Types of Solar Collector and its applications.

thermal performance of solar collectors using carbon-based nanofluids. The aim of this study is to review the heat transfer characteristics and flow properties of various solar collectors using nanofluids of graphene and carbon nanotubes (CNTs). The effect of nanofluid concentration, inlet fluid temperature and flow rates on the thermal conductivity and thermal performance of the solar collectors were discussed. Finally, the current challenges on the application of carbon-based nanofluids in solar collectors and the scope for future study were presented. 2. Method This review is a systematic literature review, which presented a profound overview of the performance of the different type of solar collectors working with carbon-based nanofluids. First, the renewable energy trend was explored in the introduction section. This was followed by a brief review of the thermophysical properties of nanofluids prepared majorly with carbon nanostructured materials such as graphene and CNT. Thirdly, reports on the different solar collectors using these specific nanofluids were selected and reviewed. The research literatures were sourced from Elsevier's' ScienceDirect, Google Scholar and ISI Web of Science. Several keywords were used for sourcing information. The keywords include nanofluids, graphene, CNTs, solar collectors, pumping power, efficiency, HTC, flat plate solar collectors, direct absorption solar collectors, evacuated tube solar collectors and PVT. More than 2 of the keywords were used. The selected literatures were peer reviewed studies and they contributed significantly to the research area. Also, Elsevier's' reference management software Mendeley was used to collate the references of the selected articles. 2.1. Thermophysical properties of carbon-based nanofluids Thermal conductivity and viscosity are very essential thermal properties to consider for the effective utilization of carbon-based nanofluids as heat exchange fluid in solar collectors. Dispersion of nanomaterials in a base fluid has been established by numerous

 et al., 2017; Mahmudul Haque et al., 2015) to researchers (Estelle significantly improve the thermal conductivity of the working fluid, which strongly influences its heat exchange characteristics. However, this positive enhancement was accompanied by an unwanted increase in viscosity (Kole and Dey, 2013; Rashmi et al., 2015). This results in an increase in pressure drop and inevitably raised pumping power. Van Trinh et al. (2018) observed an augmentation in thermal conductivity of 18% and 50% for hybrid graphene and CNTs dispersed in ethylene glycol (EG) at 303 K and 323 K respectively. Sadeghinezhad et al. (2015) reported a thermal conductivity intensification between 7.96 and 25% for graphene nanoplatelets (GNP) nanofluid with a weight loading of 0.025e0.1 wt% at 288e313 K while the viscosity increases by 9e38%. A viscosity increment of 51.2e51.5% was reported by Mehrali et al. (2016) for 0.01e0.06 wt% of nitrogen-doped graphene (NDG) suspended in water with the aid of 0.025 wt% Triton X-100 surfactant. For a similar experiment, Mehrali et al. (2016, 2014a) reported a thermal conductivity intensification between 22.15 and 36.78%. The study further revealed that viscosity and thermal conductivity is influenced not only by concentration but also by temperature. The viscosity was noted to reduce with rising temperature while the thermal conductivity increases. As presented in Fig. 4, Khosrojerdi et al. (2017) obtained a higher thermal conductivity augmentation at a higher weight fraction of graphene nanoplatelets and also at elevated temperature. The thermal conductivity augmentation could be ascribed to the intensification in Brownian motion as the nanomaterial concentration is increased. This produces a turbulent movement of particles and intensified transfer of energy between particles. Also, a thicker interfacial layer is developed by the base fluid molecules on the nanomaterial surface, thus augmenting thermal conductivity (Mishra, 2014). This is further enhanced at a higher temperature. In addition, the intermolecular forces deteriorate at a higher temperature and lessen the resistance of nanofluid to shear stress (Chai et al., 2017). This is responsible for the reduction in viscosity at high temperature. This viscosity reduction also reportedly eases and speed up the particle motion, subsequently augmenting thermal conductivity and thermal exchange property (Sajid and Ali, 2019). However, the suggestion that Brownian motion is responsible

A. Borode et al. / Journal of Cleaner Production 241 (2019) 118311

Fig. 4. Effect of Temperature and Weight Fraction of the thermal conductivity (Khosrojerdi et al., 2017).

for thermal conductivity augmentation is not generally accepted or recognized to be accurate. Other phenomena, which include clustering of nanomaterials, interfacial layer influence around nanomaterials and ballistic phonon transport in nanomaterials, have been theorized by different authors as the mechanism responsible for the enhancement in thermal energy exchange property of nanofluids. Clustering occurs when nanomaterials aggregate to form a larger particle, which subsequently creates a huge conductive path if the clusters form a percolated structure. This was reported to enhance thermal conductivity. Although, extreme clustering of nanomaterials results in settling, which subsequently reduces the augmentation in thermal conductivity. Daviran et al. (2017) conducted experimentation to evaluate the role of clustering and Brownian motion on the heat transfer mechanism of MWCNT nanofluid. The configuration of the nanofluids structure was analysed using microscopic image processing. Experimental temperature was varied between 273 K and 328 K for the evaluation of the thermal conductivity in relation to viscosity. The authors realized that varying temperature is not enough to assess the influence of Brownian motion and it is unreasonable to link to thermal conductivity augmentation to Brownian motion. This is because the random motion of fluid molecules can justify either the influence of Brownian motion or clustering phenomena. Nonetheless, in a separate study to evaluate the effect of viscosity, they observed that a highly viscous nanofluid has a higher thermal conductivity, which clearly shows that the role of Brownian motion is not significant. Furthermore, the configuration of microscopic images obtained in the study indicates the remarkable contribution of clustering. Keblinski et al. (2002) also proposed that the influence of Brownian motion on the thermal conductivity augmentation is inconsequential as the random motion of nanomaterial in nanofluid is quite slower than that of the base fluid. This was also supported by Sarkar and Selvam (2007). However, Sarkar and Selvam (2007) advocated that this unequal movement of the nanomaterial and base fluid is the main mechanism responsible for the increase in the thermal conductivity. The contrasting information as to which mechanism is responsible for heat conduction in nanofluid or which one plays a more dominant role is generating a lot of controversies. Thus, an in-depth understanding of the mechanism responsible for the enhancement in thermal conductivity is still required. The different findings on the thermal conductivity and viscosity of carbon-based nanofluids

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are outlined in Table 1. Apart from nanofluid concentration and temperature, the other considerations that influence thermal conductivity and viscosity include the morphology of the nanomaterial (Mehrali et al., 2014b; Xing et al., 2015), type of base fluid (Aravind et al., 2011), and surfactant additives (Assael et al., 2006; Sadri et al., 2014). Different researchers (Mehrali et al., 2014b; Xing et al., 2015) reported that carbon nanomaterials with higher aspect ratio and specific surface area produce a higher thermal conductivity enhancement. Also, addition of surfactants such as sodium dodecylbenzene sulfonate (SDBS), Gum Arabic (GA), Triton X-100 and cetyltrimethylammonium bromide (CTAB) were found to stabilize nanofluids and also increase the thermal conductivity and viscosity of nanofluid with exception to sodium dodecyl sulfate (SDS), which produces a poorer thermal conductivity performance in comparison to the base fluid (Sabiha et al., 2016; Walvekar et al., 2016). Finally, the thermal conductivity is also influenced by the base fluid as reported by Aravind et al. (2011) They obtained a thermal conductivity intensification of 33% and 40% for 0.03 vol% multi-walled CNT (MWCNT) added to water and EG respectively. EG is mostly used as base fluid or added to water because of its antifreeze property. It enables the application of nanofluid in cold weather region with a temperature of 273.15 K and below. As observed in the study by Aravind et al. (2011), EG-based nanofluid has a higher thermal conductivity than water-based nanofluid. This is because of the higher thermal conductivity of EG in comparison to water. In a  et al. (2017) on the thermophysical properties of study by Estelle CNT nanofluid with water and a mixture of water and EG as base fluid. The authors found that a nanofluid based on the mixture of water and EG has a thermal conductivity increase of 2.3e4.1% more than that of water for CNT loading of 0.01e0.05 wt%. Also, there was a significant increase in viscosity of 1.576e1.718 mPa s. However, in cases where the nanofluid freezes and thaw, the stability and thermophysical properties of the nanofluid were reportedly not influenced by the low temperature (Choi et al., 2019). Notwithstanding, detailed study needs to be conducted to verify the effect of low temperature on suspension stability and thermophysical properties of carbon-based nanofluid for application in cold regions. 3. Thermal application of carbon-based nanofluids in solar collectors In this section, the literature survey on the performance of different solar collectors utilizing graphene nanofluids and CNTs nanofluids are presented. The solar collectors in consideration are flat plate solar collector, evacuated tube solar collector, direct absorption solar collector, parabolic trough solar collector and hybrid photovoltaic thermal collector. 3.1. Flat plate solar collector (FPSC) FPSC is the most basic and most researched solar thermal collector for domestic or commercial water or space heating system. It consists of (i) a black solar absorption surface, (ii) transparent glazing cover, which is glass or plastic, prevents heat loss from the surface due to radiation and convection, (iii) tubes to transport heat from the collector and (iv) an insulated wooden or metal box to reduce heat losses. FPSC functions by collecting heat energy from the absorption of the solar energy incident on the black surface through the glazing and transport the heat through fluids such as water or air for use (Orosz and Dickes, 2017). Fig. 5 displays the schematics of an FPSC. Numerous studies are available on the utilization of carbon-based nanofluids in FPSC with respect to the concentration of nanofluid, nanofluid pH, surfactant and flow rates.

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Table 1 Summary of the findings on the thermophysical properties of carbon-based Nanofluids. Author

Nanomaterial

Base Fluid

Concentration Findings

Water þ SDS

0e0.3 vol%

Water

1.1e0.02 wt%

  Sadeghinezhad et al. GNP Distilled Water 0.025e0.1 wt  (2015) (DW) %  Huang et al. (2015) MWCNT Water 0.01111  e0.0555 vol%  Anin Vincely and Graphene oxide De-ionized 0.005  Natarajan (2016) (GO) water (DIW) e0.02 wt%  Said et al. (2015b)

SWCNT

Ahmadi et al. (2016) GNP

EG 0.2e1.0 vol% Water 0.01e0.2 vol% DIW þ Chitosan 0.25e1.5 wt%

Thermal conductivity enhancement of 12e91% at 298e323 K Maximum increment of 39% in viscosity 7.96e25% thermal conductivity intensification 9e38% increase in viscosity 0.07e0.42% increase in thermal conductivity 12.99e20.66% elevation in viscosity Thermal conductivity enhancement of 11.2e19.3% and 16.5e24% at 30e70  C (303e343 K) for 0.01 and 0.02 wt% nanofluid concentration Viscosity reduction of 9.8% for 0.05 vol% nanofluid at temperature increase of 30e50  C (303 e323 K)  11.37e38.63% intensification in thermal conductivity  Maximum increase of 7.52% in kinematic viscosity at 30e60  C (303e333 K)  Maximum thermal conductivity augmentation of 37%  Maximum increase of 51.2e51.5% in viscosity  14.8, 25 and 27.64% enhancement in thermal conductivity for nanofluid with 0.1 wt% graphene of specific surface area of 300, 500, 750 m2/g respectively  Maximum increase of 44% in viscosity  8.7e18.9% increase in thermal conductivity at 25e40  C (298e313 K) 60% increase in viscosity for 0.1 wt% nanofluid at 25  C (298 K)  1.7e21.2% augmentation in thermal conductivity  200% enhancement in viscosity  4.14e7.98% and 3.10e4.98% enhancement in thermal conductivity of SWCNT and MWCNT nanofluids respectively at 10e60  C (283e333 K)  1.6e12.4% thermal conductivity enhancement  Maximum thermal conductivity enhancement of 27%  Maximum increase of 233% in viscosity

DIW

0.1e0.3 wt%

 56% augmentation in thermal conductivity for 0.3 wt% MWCNT loading

EG

0.0175 e0.07 vol% 0.2e0.6 vol%

 Thermal conductivity enhancement of 18e50%

0.041 e0.395 vol% 0.01 wt% 0.01e0.05 wt %

       

Mehrali et al. (2016) Nitrogen-doped DW þ Triton X- 0.01e0.06 wt graphene 100 % Mehrali et al. (2014b) GNP DW 0.025e0.1 wt % Esfahani et al. (2016) GO

Water

0.01e0.1 wt%

Wang et al. (2018)

GNP

0.01e1.0 wt%

Xing et al. (2015)

SWCNT MWCNT MWCNT Graphene MWCNT

Water þ EG (50:50) Water

Liu et al. (2005) Gandhi et al. (2011) Hung and Chou (2012) Sarafraz et al. (2016) Functionalized MWCNT Van Trinh et al. GN-CNT hybrid (2018) Poongavanam et al. MWCNT (2019) Kole and Dey (2013) Graphene nanosheet Rashmi et al. (2015) CNT  et al. (2017) CNT Estelle

Khosrojerdi et al. (2017)

GONP

Glycol þ GA Water þ EG Water þ GA Water Water þ EG

Water

0.48 vol%

0.001 e0.045 wt%

 28.45e30.59% augmentation in thermal conductivity between 30 and 50  C (303e323 K) Thermal conductivity enhancement of approx. 15% for 0.395 vol% nanomaterial loading Approx. 100% increase in viscosity at room temperature Thermal conductivity enhancement of 4e125.6% Thermal Conductivity enhancement of 10.9e11.3% Viscosity of 0.604e0.632 mPa s Thermal Conductivity enhancement of 13.2e15.4% Viscosity of 2.18e2.35 mPa s Thermal conductivity enhancement of approx. 1e11%

Yousefi et al. (2012a) conducted an experiment to assess the effects of different concentrations (0.2 wt% and 0.4 wt%) of MWCNT nanofluid on the thermal efficiency of an FPSC. They also investigated the influence of Triton X-100 surfactant in the nanofluid on

the thermal performance of FPSC. The experimental results revealed that nanofluid loaded with 0.2 wt% MWCNT without surfactant reduces the efficiency of FPSC in comparison to water while an increase in MWCNT loading in base fluid from 0.2 wt% to

Fig. 5. Schematics of an FPSC (Hawwash et al., 2018).

A. Borode et al. / Journal of Cleaner Production 241 (2019) 118311

0.4 wt% leads to a remarkable increase in the efficiency of the FPSC when compared to water. However, the addition of Triton X-100 to nanofluid with 0.2 wt% MWCNT enhances the efficiency, but at a point, it became somewhat detrimental to heat transfer rate due to the high foaming attributes of the surfactant. The research group (Yousefi et al., 2012b) also assessed the effect of different pH (3.5, 6.5 and 9.5) of 0.2 wt% MWCNT/water nanofluid on the thermal efficiency of FPSC. They found an additional enhancement in FPSC efficiency for increase or decrease in the pH of nanofluid to that of isoelectric point (pH ¼ 7.4). This result shows that in addition to nanofluid concentration, pH value also affects the thermal performance of a nanofluid in FPSC. Mass flow rate is another parameter whose effect was investigated in the previous study (Yousefi et al., 2012a). They reported an increase in efficiency with an increase in mass flow rate from 0.0167 to 0.05 kg/s. Based on the experimental results by Yousefi et al. (2012a), Faizal et al. (2013) investigated the potential of reducing the size of FPSC with the application of MWCNT nanofluid as working fluid. They reported a 37% diminution in the original size of FPSC with the application of MWCNT based nanofluid, thus minimizing the total cost of the thermal system. This clearly proves that the application of carbon-based nanofluid can be used to achieve the miniaturization of heat collecting or exchanging components. Vijayakumaar et al. (2013) conducted experimentation to determine the effect of CNT/water nanofluid in FPSC. They found a 39% enhancement in efficiency for using 0.5 wt% CNT nanofluid when compared to water. Also, Ekramian et al. (2014) conducted a numerical investigation into the heat transfer performance of FPSC using nanofluids of various nanomaterials (MWCNT, alumina and CuO) at different concentrations (1,2 and 3 wt%). The study investigated the influence of temperature and mass flow rate on the efficiency of FPSC. The numerical results were validated with an experimental study. They revealed that raising the mass flow rate reduces the fluid outlet temperature with the absorber temperature and improves the efficiency of FPSC. Thermal efficiency of 13.2% was reported for raising the mass flow rate from 0.03 kg/s to 0.05 kg/s for nanofluid with 1% MWCNT loading. However, an increase in fluid inlet temperature was found to reduce the temperature difference between the absorber plate and glass cover. This increases the thermal loss in the collector, consequently reducing the thermal efficiency of FPSC. Further, they reported that the heat transfer coefficient (HTC) and the efficiency of using CuO nanofluid are higher than that of MWCNT and alumina nanofluids. This was despite the higher thermal conductivity of MWCNT in comparison to other nanomaterials. The reason attributed to the lower thermal performance of MWCNT nanofluid was its higher specific heat in comparison to that of CuO. Also, the authors stated that viscosity enhancement is another factor that notably influences the HTC. This is because a huge viscous force can overpower the Brownian movement of particles, consequently increasing the thermal boundary layer thickness, which in turn reduces HTC. Said et al. (2014) theoretically determined the heat transfer characteristics and pressure drop of a FPSC using single-walled CNT (SWCNT) nanofluid as the absorbing medium under laminar flow condition. An increase in volume fraction was found to increase HTC. They also observed an increase in the HTC and Nusselt number with a negligible increase in pumping power and pressure drop as the flow rate increases. A maximum HTC enhancement of 15.33% and a 4.34% reduction in entropy generation was reported in comparison to water. The research group (Said et al., 2015b) also investigated the thermal efficiency of an FPSC using SWCNT/water nanofluid dispersed with SDS surfactant. The nanofluids were prepared by suspending 0.1 vol% and 0.3 vol% of SWCNT in water with the aid of 0.1 vol% and 0.3 vol% SDS surfactant. They were reportedly stable for at least 30 days. The study observed an

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enhanced overall thermal performance by increasing the inlet fluid temperature more than the ambient temperature and using a low flow rate. The experimental outcomes revealed a maximum exergy efficiency of 26.25% for FPSC using nanofluids of 0.3 vol% SWCNT at a flow rate of 0.0083 kg/s when compared to that of water with 8.77% efficiency. Members of the group (Said et al., 2015a) theoretically evaluated the thermal performance of an FPSC operating with graphene nanofluids of two different base fluids (acetone and water). The study revealed that dispersing a minimal amount of graphene in water enhances the exergy efficiency by 21% and reduces the entropy generation by 4% in contrast to the base fluid. Furthermore, the exergy efficiency was observed to increase as the volumetric flow rate increases for all the nanofluids. However, this report tends to produce a conflicting result as to the suppressing performance of SDS on the thermal performance of nanofluid as reported by some authors (Mahmudul Haque et al., 2015; Sadri et al., 2014). Notwithstanding, the improved stability of nanofluid dispersed with the aid of SDS agrees with some other authors (Sharma et al., 2018). Anin Vincely and Natarajan (2016) conducted an experimental investigation of an FPSC using GO nanofluid under forced circulation with the aid of a pump. The nanofluids were prepared by suspending different mass concentrations (0.005, 0.01 and 0.02) of graphene in de-ionized water. The nanofluids were found to be stable for 60 days without agglomeration. Under laminar flow conditions, the overall HTC, friction factor and efficiency of the solar collector were evaluated. The experimental results presented a maximum thermal efficiency of 7.3% for FPSC using nanofluid with GO loading of 0.02 vol% at a mass flow rate of 0.0167 kg/s in comparison to FPSC using water. Overall HTC enhancement of 8.03%, 10.93% and 11.5% were reported for the nanofluids with GO mass loading of 0.005, 0.01 and 0.02 respectively. This established that solar collector efficiency is higher at higher nanomaterial loading. Ahmadi et al. (2016) assessed the thermal performance of FPSC utilizing Graphene nanoplatelets (GNP) based nanofluids. The nanofluid samples were prepared by suspending 0.01 and 0.02 wt% of GNP into water. The schematics of the experimental set-up is shown in Fig. 6. The experimental result revealed a thermal efficiency increase of up to 12.189% and 18.87% for FPSC using nanofluids with GNP loading of 0.01 and 0.02 wt% respectively when compared to water at a mass flow rate of 0.015 kg/s. The outlet fluid temperature of the water heater was reported to reach 340.5 K and 344 K for nanofluid with GNP loading 0.01 and 0.02 wt% respectively. In addition, the study calculates the theoretical thermal efficiency and the results agreed with the experimental thermal efficiency. These outcomes established that nanofluids with higher GNP concentration produce a higher increase in collector efficiency. Vakili et al. (2016b) assessed the performance of a volumetric solar collector for domestic hot water system utilizing GNP nanofluid of different weight concentrations (0.0005, 0.001 and 0.005). The experimental assessments were carried out at inlet temperature range of 308 Ke313 K and flow rates of 0.0075, 0.015 and 0.225 kg/s. The study revealed an enhancement in the collector efficiency as the GNP weight concentration and mass flow rate increases. The efficiency was found to increase as the mass flow rate increases from 0.0075 to 0.015 kg/s. This efficiency reduces when the mass flow rate increases from 0.015 to 0.225 kg/s. Thus, the maximum zero-loss efficiencies were found to be 83.54%, 89.71% and 93.24% for GNP nanofluids with weight concentration of 0.0005, 0.001 and 0.005 respectively at an optimum mass flow rate of 0.015 kg/s, while that of base-fluid is 69.96%. This study proves that carbon-based nanofluids have good thermal properties for application in domestic water heating system. However, there is no report as regards the stability of the nanofluid. Verma et al. (2017) assessed the thermal performance of FPSC

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Fig. 6. Experimental Set-Up to evaluate FPSC using GNP based Nanofluids (Ahmadi et al., 2016).

using varieties of nanofluids. The nanofluids include MWCNT, Graphene, CuO, alumina, TiO2 and SiO2 based nanofluids. The noted an increase in the collector efficiency for all the nanofluids when the intensity increases from 400 to 1200 W/m2. The influence of nanofluid concentration and flow rates were evaluated as displayed in Fig. 7 and Fig. 8. Fig. 7 reveals that the entropy generation rate increases while the exergy efficiency reduces when the mass flow rate was increased from 0.01 to 0.05 kg/s. However, the collector efficiency increases with an increase in flow rate up to 0.025 kg/s, after which it starts reducing slightly. As presented in Fig. 8a, the authors observed an increase in collector efficiency when the nanomaterial loading was raised up to 0.75 vol%, after which it bns to slightly decline. However, a higher pumping power was noticed

at higher nanomaterial loading as presented in Fig. 8b. This indicates that the optimum mass flow rates and particle loading is 0.025 kg/s and 0.75 vol% respectively. At this optimum parameter, they realized an enhancement of 29.32%, 21.46%,16.67%, 10.86%, 6.97% and 5.74% in exergy efficiency for MWCNT, Graphene, CuO, alumina, TiO2 and SiO2 based nanofluids respectively. The entropy generation was found to reduce by 65.55%, 57.89%, 48.32%, 36.84%, 24.49% and 10.04% for MWCNT, Graphene, CuO, alumina, TiO2 and SiO2 based nanofluids respectively. In addition, the enhancement in energetic efficiency was reported to be 23.47%, 16.93%, 12.64%, 8.28%, 5.09% and 4.08% for MWCNT, Graphene, CuO, Alumina, TiO2 and SiO2 based nanofluids respectively at optimum operating conditions. The experimental outcomes prove that MWCNT

Fig. 7. Effect of mass flow rate on the (a) Collector efficiency (b) Entropy generation (c) Exergy Efficiency (Verma et al., 2017).

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Fig. 8. Effects of nanomaterial concentration on the (a) Collector efficiency (b) Pumping power loss ratio (Verma et al., 2017).

followed by graphene have the best thermal performance. This indicates that a reduction in the FPSC surface area could be achieved with the utilization of MWCNTs and graphene nanofluids as the collector fluid instead of the traditional working fluids, thus minimizing the manufacturing cost. The percentage reduction achieved with the nanofluids at optimum operating parameters is presented in Fig. 9. However, this study outcome contradicts the study by Ekramian et al. (2014). This report clearly proves that MWCNT and graphene is a better nanomaterial for application in the solar collector than other nanomaterials including CuO. Although, the stability of the nanomaterials cannot be deduced from the report. Sadripour (2017) conducted a numerical study on the thermal performance of a heat sink FPSC with insulator mixers utilizing MWCNT nanofluid. The study investigated the effects of Reynolds number between 50 and 12000, solar intensity range of 700e1000 W/m2 and different nanofluid concentration on the thermal performance of FPSC. The study revealed a maximum enhancement of 12.7% in exergy efficiency for the highest volume concentration of 0.1% and the highest mass flow rate of 0.055 kg/s. Further, the exergy efficiency was found to increase with inlet nanofluid temperature. This result reveals that the exergy efficiency is increased with an increase in the flow rate and nanomaterial concentration. However, the exergy efficiency will improve with an

Fig. 9. Percentage reduction in FPSC area (Verma et al., 2017).

increase in the inlet fluid temperature irrespective of the fluid used. The same can also be stated for the influence of increase in mass flow rate for any working fluid. This indicates that addition and increase in nanomaterials concentration, which results in an improvement in thermal conductivity, is the most important factor that contributes a major change to the thermal collecting performance of a solar collector. Recently, Bioucas et al. (2018) investigated the effect of nanographene-based nanofluid in an FPSC with different radiation sources. The radiation sources include 1000 W halogen lamp and Sunlight. The nanofluids used were prepared by dispersing different concentrations of nanographene into a 70:30 vol/vol mixture of water and EG without surfactants. The studies with sunlight radiation source displayed better outcomes. Although FPSC using nanofluids performed better than water under all the radiation sources without traceable clogging issue. Maximum thermal efficiency of 60.56% was reported for 0.10 wt% nanographene based nanofluid. This represents a 5.90% enhancement when compared with water. Most recently, in a bid to enhance the efficiency of an FPSC, Eltaweel and Abdel-Rehim (2019) replaced the conventional working fluid with MWCNT based nanofluid. The influence of this nanofluid was investigated. The experimental results revealed that by suspending 0.01 wt%, 0.05 wt% and 0.1 wt% of MWCNTs in distilled water, it improved the efficiency of the collector by 16%, 21% and 34.13% respectively. Further study showed that using forced circulation at a flow rate of 1.5 L/min also increased the efficiency of the FPSC by 6.21% in contrast to what was attained merely by using thermosiphon. Maximum exergy efficiency of 23.35% was reported for the FPSC utilizing MWCNT nanofluid in comparison to that using water, which is 14.55%. This indicates that the application of carbon-based nanofluid in FPSC can significantly enhance its thermal efficiency. Based on the different studies in this section, carbon-based nanofluid is a promising working fluid for improving the performance of the solar collector. However, an interesting situation was observed by Vakili et al. (2016b) and Verma et al. (2017), where the FPSC efficiency starts declining at a mass flow rate more than 0.015 kg/s and 0.025 kg/s respectively. This could be attributed to the concomitant increase in pressure drop or pumping power as the mass flow rate increases. It was also observed that most of the studies only focused on using low nanomaterial concentrations up to 0.75 vol% with the exception of Ekramian et al. (2014). However, the study contradicted another literature (Verma et al., 2017), as CuO based nanofluid was found to perform better than MWCNT nanofluid. The justification for this contrasting result could be the poor stability of MWCNT nanofluid at higher concentration.

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Notwithstanding, more studies need to be conducted to verify the performance of carbon-based nanofluids with high nanofluid concentrations. Also, the long-term stability of the nanofluid in operation still needs to be determined coupled with the effects of different surfactants on the collector performance. Table 2 presents the summary of the major findings of FPSC using carbon-based nanofluids. 3.2. Evacuated tube solar collector (ETSC) ETSC uses an absorber embedded in a glass tube to collect solar energy. The glass tube surrounding the absorber achieves a very high vacuum and can withstand atmospheric pressure while also minimizing heat loss due to convection and conduction. The absorber could be a metal or a concentric glass tube. Heat is transported through a working fluid that flows through the tube or makes contact with a heat pipe inside the tube. Heat can also be transported from the heat pipes to the working fluid in a manifold (which is a heat exchanger) perpendicularly placed to the tubes. Fig. 10 depicts the schematics of a typical evacuated tube. Different authors have reported an improvement in the performance of an ETSC using carbon-based nanofluid. Sabiha et al. (2015) conducted experimentation to evaluate the thermal efficiency of an ETSC utilizing SWCNT/Water nanofluid. The nanofluids were prepared by dispersing different concentrations (0.05, 0.1 and 0.2 vol%) of SWCNTs into distilled water with

Fig. 10. A schematic of an arrangement of the evacuated tubes (Iranmanesh et al., 2017).

SDS surfactant to make it stable. A comparison between the thermal performance of ETSC using SWCNTs nanofluid and water was reported at different mass flow rates of 0.0008, 0.017 and 0.025 kg/ s. The experimentation revealed an enhancement in the performance of ETSC using SWCNTs nanofluids in comparison to water at all mass flow rates. They observed an increase in the thermal

Table 2 Summary of findings on FPSC using carbon-based nanofluids. Author

Nanomaterial

Yousefi et al. (2012b) Yousefi et al. (2012a) Faizal et al. (2013) Vijayakumaar et al. (2013) Ekramian et al. (2014)

MWCNT þ Triton X-100 Water

Said et al. (2014)

Said et al. (2015b) Anin Vincely and Natarajan (2016) Vakili et al. (2016b) Ahmadi et al. (2016) Sadripour (2017) Verma et al. (2017)

Bioucas et al. (2018) Verma et al. (2018)

Eltaweel and AbdelRehim (2019)

Base Fluid

Concentration Findings

 Improvement in thermal efficiency with an increase or decrease in pH in relation to that of the isoelectric point MWCNT þ Triton X-100 Water 0.2e0.4 wt%  Improvement in collector efficiency with the addition of surfactant, an increase in flow rate and increase in MWCNT concentration MWCNT þ Triton X-100 Water 0.2 wt%  37% reduction in size of FPSC CNT þ Polysorbate 80 Water 0.40e0.60 wt  39% increase in efficiency for 0.5 wt% nanofluid when compared to water % MWCNT Water 1e3 wt%  Increase in flow rate increases thermal efficiency Al2O3  Raising the flow rate from 0.03 to 0.05 kg/s increases the efficiency of a collector using 1 wt% MWCNT by 13.2% CuO SWCNT Water 0.02e0.034  Entropy generation diminishes by 4.34% vol%  Enhancement in HTC by 15.33%  Pumping power increases by 1.20% in comparison to the collector using water SWCNT þ SDS Water 0.1e0.3 vol%  Maximum exergy efficiency and energy efficiency of 26.25% and 95.12% respectively GO DIW 0.005  8.03e11.5% increase in HTC e0.02 wt%  Collector efficiency enhancement of 7.3% for using 0.2 vol% nanofluid at flow rate of 0.0176 kg/s GNP DIW 0.0005 Zero loss efficiency of 83.5e93.2% at flow rate of 0.015 kg/s e0.005 wt% Collector efficiency increases with nanofluid weight fraction GNP Water 0.01e0.02 wt  Increase of up to 18.87% in thermal efficiency %  Water was heated up to 71  C (344 K) MWCNT Water 0e0.1 vol%  Maximum increase of 12.78% in exergy efficiency for FPSC using nanofluid with the highest concentration of 0.1 vol% and highest mass flow rate of 0.055 kg/s MWCNT, Graphene, Water 0.75 vol%  Maximum reduction in entropy of 65.55% and 57.89% for MWCNT nanofluid and Graphene Al2O3, CuO, TiO2, SiO2 nanofluid respectively  Maximum energy efficiency of 23.47% and 16.97%  Maximum exergy efficiency of 29.32% and 21.46% Graphene Water þ EG 0.05e0.10 vol  Maximum efficiency enhancement of 5.90% (70:30) % MgO-MWCNT, CuOWater 0.25e2 vol%  Exergetic efficiency of 71.54% and 70.63% collector using MgO hybrid and CuO hybrid MWCNT, MWCNT nanofluids respectively  Energetic efficiency of 70.55% and 69.11% collector using MgO hybrid and CuO hybrid nanofluids respectively  MgO hybrid nanofluids perform better than CuO hybrid nanofluids  Exergetic and energetic efficiency of FPSC using MgO hybrid nanofluid enhances by 25% and 16.28% with respect to water and MgO nanofluid respectively MWCNT DW 0.01e0.1 wt%  Increase in energy efficiency by 16e34.13% for FPSC using 0.01e0.1 wt% nanofluid in comparison to water  Maximum exergy efficiency of 23.35% for 0.1 wt% MWCNT nanofluid  34% reduction in the size of FPSC using 0.1 wt% nanofluid. 0.2 wt%

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efficiency of the collector with an increase in the volume concentration of SWCNT and mass flow rates. Maximum efficiency of 93.43% was reported for the highest volume concentration (0.2 vol %) of SWCNTs nanofluid and the highest mass flow rate (0.025 kg/s). This study shows that SWCNTs nanofluids can be used as the heat transfer fluid in an ETSC to more efficiently absorb solar radiation heat and convert solar energy into thermal energy in comparison to water. Tong et al. (2015) investigated the influence of MWCNT/water nanofluid as the working fluid on the thermal performance an enclosed-type evacuated U-tube solar collector. The nanofluids were prepared by suspending an MWCNTs with concentrations of 0.06, 0.12, 0.18 and 0.24 vol% into water with 0.25 wt% GA as the dispersant. Results revealed that the application of MWCNT nanofluid enhances the efficiency of the collector by at least 4%. SWCNT nanofluid with the highest concentration of 0.24 vol% was found to produce the maximum enhancement of 8% in HTC in comparison to water. The study further revealed that 50 solar collectors with MWCNTs nanofluids have the potential to replace 615 kg of coal annually, thus minimizing CO2 and SO2 emission by 1600 kg and 5.5 kg respectively. This experimental outcome indicates that performance enhancement with the use of nanofluid can help in reducing greenhouse effects. Kim et al. (2016) theoretically determined the performance of a U-tube solar collector utilizing different nanofluids. The nanofluids include 0.1e0.2 vol% MWCNT, 1e3 vol% Al2O3, 1e3 vol% CuO, 1e3 vol% TiO2 and 1e3 vol% SiO2 suspended in 20% propylene glycol-water. The predicted results revealed the thermal efficiency enhancement for the different heat transfer fluid in increasing order of SiO2, TiO2, Al2O3, CuO and MWCNT nanofluids at different temperature difference as shown in Fig. 11. The maximum thermal efficiency of 62.8% was reported for 0.2 vol% MWCNT nanofluid. This represents a 10.5% enhancement in comparison to the base fluid. This indicates that low concentration of MWCNT nanofluid is enough to enhance the performance of the collector, which is not feasible with the other nanofluids. However, as shown in Table 3, all the nanofluids could significantly reduce energy consumption and consequently reduce CO2 and SO2 emission from coal usage. However, the performance of a lower concentration of MWCNT nanofluid in comparison to CuO nanofluid clearly invalidates the study by Ekramian et al. (2014) and supports Verma et al. (2017) This indicates that the poor performance of MWCNT nanofluid against CuO nanofluid in the study by Ekramian et al. (2014) could be attributed to poor nanofluid stability. Notwithstanding, this creates a research gap that needs to be filled. Thus, a detailed study is required. Iranmanesh et al. (2017) conducted an experimental investigation to study the influence of GNP/water nanofluid on the heat transfer performance of ETSC-based water heater. The assessment of the thermal efficiency of the collector was conducted at different

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flow rates of 0.5, 0.1 and 1.5 L/min. Fig. 12 displays the set-up for this experiment. As displayed in Fig. 13, the study established that raising the GNP weight concentration in base fluid results in an increase in thermal energy gain and outlet temperature. A maximum collector efficiency of 90.7% was reported when GNP nanofluid with the highest concentration of 0.1 wt% was used as the heat transfer working fluid at the highest flow rate of 1.5 L/min. This represents a 35.8% enhancement in thermal efficiency when compared to distilled water. This displays the remarkable thermal properties of GNP nanofluid. In addition, the maximum temperature difference was reported for GNP nanofluid concentration of 0.1 wt% at a flow rate of 0.5 L/min. To improve the thermal performance of an ETSC, Mahbubul et al. (2018) employed SWCNT nanofluid as the heat transfer fluid. They observed a higher thermal efficiency at higher solar irradiance as displayed in Fig. 14. The influence of nanofluid on the performance of the collector was analysed and compared with that of water. Thermal efficiencies of 56.7% and 66% were reported for ETSC working with water and 0.2% SWCNT nanofluid respectively. This indicates the nanofluid enhances efficiency by 10% when compared to water. However, despite the study revealing a remarkable heat transfer performance of the nanofluid, the authors didn't present any information about the stability of the nanofluid and the effects of the nanofluid on the pumping power of the system. Recently, Natividade et al. (2019) evaluated the thermal efficiency of an ETSC equipped with a parabolic concentrator using low volume concentration of multilayer graphene (MLG)-based nanofluids. The assessment of the efficiency was conducted at various flow rates (0.1, 0.4, 0.7 and 1 L/min) for nanofluids of two volume concentrations (0.00045 vol% and 0.00068 vol%) with and without the concentrator. The results as shown in Fig. 15 indicates that the use of the concentrator augments the thermal efficiency remarkably by approx. 298% in comparison to the collector without concentrator. Further, the best thermal performance was observed at a flow rate of 0.7 L/min. Thermal efficiencies of 31% and 76% were reported for the collector using MLG nanofluids with concentrations of 0.00045 vol% and 0.00068 vol% respectively when compared to the base fluid. The diminution in thermal efficiency at a flow rate above 0.7 L/min could be explained by a possible relative increase in pressure drop. However, this could not be verified from the study because there was no information about the pressure drop and pumping power penalty. The different research studies (Iranmanesh et al., 2017; Kim et al., 2016; Mahbubul et al., 2018; Sabiha et al., 2015; Natividade et al., 2019) on ETSC using carbon-based nanofluids clearly shows that the collector efficiency is increased from 31% to 93.43% with low nanomaterial concentrations of up to 0.2 vol%. Apart from having the potential the replace conventional base fluids in ETSC, carbon-based nanofluids can also assist in reducing our carbon footprint by replacing coal usage (Kim et al., 2016; Tong et al., 2015). However, there is limited information on the pumping power penalty attributed to the use of carbon-based nanofluid, which possesses high viscosity. Also, the stability of the nanofluids was not reported by most of the authors. Thus, detailed studies need to be done to fully assess the performance and flow properties of ETSC using carbon-based nanofluids. Also, the effects of surfactants on these performance needs to be determined. Major findings on the performance of ETSC using carbon-based nanofluids are presented in Table 4. 3.3. Direct absorption solar collector (DASC)

Fig. 11. Thermal efficiency of the Collector at different nanofluid concentrations and thermal loss of (a) 0 and (b) 0.15 (Kim et al., 2016).

DASC utilizes fluid as the absorbing medium for incident sunlight as against a solid absorber. The fluid could be either liquid or gas. Fig. 16 depicts a schematic of a typical DASC. Different carbon

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Table 3 Energy savings from weight reduction of Coal, CO2 and SO2 (Kim et al., 2016). Nanofluid

Concentration (vol%)

Weight of coal (kg)

Weight of CO2 (kg)

Weight of SO2 (kg)

MWCNT CuO Al2O3 TiO2 SiO2 20% PG-water

0.2 3.0 3.0 3.0 3.0 e

792.1 759.5 753.0 713.5 700.2 660.8

2083.1 1997.5 1980.3 1876.5 1841.6 1737.8

6.7 6.5 6.4 6.1 6.0 5.6

Fig. 12. A typical set-up of an evacuated tube solar collector (Iranmanesh et al., 2017).

Fig. 13. Effect of weight concentration and volumetric flow rate on (a) temperature difference (b) thermal efficiency (Iranmanesh et al., 2017).

nanomaterials have been dispersed in the working fluid to improve its absorption properties and performance efficiency. Otanicar et al. (2010) evaluated the efficiency of a DASC utilizing nanofluids of three different nanomaterials. These nanomaterials include CNTs, graphite and Silver. A 5% enhancement in efficiency was reported for utilizing nanofluids. The results showed an initial substantial increase in efficiency with increase in volume fraction up to 0.5 vol%, after which additional increase causes a slight decrease in efficiency. This diminution in efficiency could be ascribed to the aggregation of nanomaterials at concentrations above 0.5 vol%. Ladjevardi et al. (2013) evaluated the direct absorptions of solar radiation in a volumetric solar collector using graphite based nanofluids. The result revealed the possibility of achieving 50% absorption of incident irradiation energy simply by using graphite nanofluids with a low volume concentration of 0.000025 vol% in comparison to water, which has a 27% absorption. Luo et al. (2014) numerically assessed the performance of a DASC using various nanofluids (graphite, long and short CNTS, Ag, Al2O3, SiO2 and Cu). The results revealed that the nanofluids enhanced the fluid outlet temperature by 30e100 K while the collector efficiency improves by 2e25% when compared to the base fluid. The study established that even low concentration of graphite nanofluid has good solar radiation absorption. Thus, it has the potential to enhance outlet temperatures and system efficiency. A photothermal efficiency of 122.7% was reported for 0.01 vol% graphite nanofluid. Hordy et al. (2014) investigated the solar energy absorption characteristics of nanofluids in a DASC. The nanofluids examined were prepared by adding plasma-functionalized MWCNTs to water, EG, Polyethylene glycol (PG) and Therminol VP-1. Even at low MWCNT loading in all the base fluids, the nanofluids were found to

A. Borode et al. / Journal of Cleaner Production 241 (2019) 118311

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Fig. 14. Effect of Solar irradiance on Solar collector efficiency (Mahbubul et al., 2018).

Fig. 15. Mean thermal efficiency of the collector using water and MLG nanofluids of different concentrations with and without parabolic concentrator (Sampaio et al., 2019).

achieve about 100% solar energy absorption. The examined glycolbased nanofluids were found to be stable at elevated temperature of 358 K and 443 K over a long-term of up to 243 days while the water-based nanofluid display moderate agglomeration. This indicates that long-term stability can be achieved at elevated temperature with glycols as the base fluid. Delfani et al. (2016) carried out an experimental and numerical study to investigate the performance characteristics of a DASC using MWCNT nanofluids at various flow rates (0.0150e0.025 kg/s) and in two different internal surfaces (black and reflective). The nanofluids were prepared by dispersing different concentrations of MWCNTs in water (70% vol/vol) and EG (30% vol/vol) mixture. The thermophysical properties were measured experimentally and were used to model the nanofluid-based DASC. The numerical

study of the performance of the DASC using nanofluid was done by using a model developed by solving the radiative transport equation and energy equation. The numerical model was used to analyse the influence of internal emissivity of the bottom wall, collector height, flow rate and nanofluid concentration on the fluid outlet temperature of the DASC. The outcome of the numerical study was found to be accurate by ±5% of the experimental results. The outcome revealed enhancement in the collector efficiency with increase in the nanofluid concentration and flow rates. MWCNTs nanofluid reportedly enhances the collector efficiency by 10e29% more than the base fluid. Further, the collector efficiency is higher when the black (absorptive) internal surface is used in comparison to the reflective surface. This is because black surfaces absorb almost all solar light incident on it, thus increasing the heat energy. Vakili et al. (2016a) investigated the optical characteristics of GNP nanofluid for application in low-temperature DASC. The nanofluids for the test were prepared by suspending 0.00025, 0.0005, 0.001 and 0.005 wt% of GNP in de-ionized water. The effect of nanofluid concentration and temperature on the optical characteristics was evaluated. From the experimentation, the optical absorption was observed to increase while the transmittance decreases as the nanofluid concentration increases. GNP nanofluid with a weight concentration of 0.005 was found to completely absorb solar energy at the wavelength of 286 nm. Increase in nanofluid concentration was also found to minimize the collector height in the DASC such that 0.005 wt% and 0.00025 wt% GNP nanofluids can totally absorb solar energy at collector height of 0.02 m and 0.10 m respectively. From the experiment, it can be established that increasing the GNP nanofluid concentration increases the coefficient of absorption and conductivity of the nanofluid. Thus, it enhances the thermal performance of collectors. Gorji and Ranjbar (2016) performed a numerical and experimental study on a DASC using different nanofluids of graphite, magnetite and silver as the absorbing medium. Surface modification of the nanomaterials was carried out to enhance their dispersibility in water. The optical properties of the DASC using nanofluids were evaluated using experimentation. The radiative

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Table 4 Summary of findings on ETSC using Carbon-based nanofluids. Author

Nanomaterial

Base Fluid

Concentration

Findings

Sabiha et al. (2015)

SWCNT

Water

0.05e0.2 vol%

Tong et al. (2015)

MWCNT þ GA

Water

0.06e0.24 vol%

Kim et al. (2016)

MWCNT, CuO, Al2O3, SiO2, Propylene TiO2 Glycol GNP Water

0.025e0.1 wt%

 Maximum efficiency of 93.43% for 0.2 vol% SWCNT nanofluid at maximum flow rate of 0.025 kg/s  Efficiency increases with nanofluid concentration and mass flow rate  Atleast 4% increase in collector efficiency  615 kg of coal is saved with the use of 50 nanofluid based ETSC  HTC increases with an increase in Re  Collector efficiency of 62.8%, which is an augmentation of 10.5% in comparison to water  Maximum efficiency of about 90.7% at flow rate of 1.5 L/min for 0.1 wt% GNP loading

SWCNT

Water

0.2 vol%

 Thermal efficiency of 66%

MLG

Water

0.00045 e0.00068%

 Increase in flow rate improves the mean thermal efficiency  Increase in thermal efficiency of the collector by 31e76% in comparison to that of water

Iranmanesh et al. (2017) Mahbubul et al. (2018) Natividade et al. (2019)

0.2 vol%

Fig. 16. Schematic of a DASC (Qin et al., 2017).

transfer in particulate media and heat transfer equations were solved using a developed computational numerical model. Also, the temperature distribution of the nanofluid within the collector was evaluated with respect to the absorption and scattering within the nanofluid medium. Concurrently, an experimental study was conducted to determine the influence of solar flux intensity, nanofluid volume fraction and flow rate on the thermal performance of the DASC and to also validate the numerical model. The result revealed an enhancement of approximately 49% and 18% in the thermal and exergy efficiencies of the collector using graphite nanofluid when compared to the base fluid at a mass flow rate of 2.5 ml/min and solar flux of 1000 W/m2. The predictions of the numerical model were found to agree with the experimental outcomes. The numerical study revealed that the outlet temperature of DASC increases with increase in the nanofluid concentration at a constant flow rate and solar flux. Moreover, the experimental study established that an increase in the nanofluid concentration and flow rate enhances the thermal efficiency of the collector. All these outcomes proved the potency of nanofluids as an absorbing medium in DASC. Wang et al. (2017) evaluated the improvement in the photothermal conversion of a graphene-based heat transfer oil used as an absorbing medium in a DASC. A high absorption coefficient, high extinction coefficient and low scattering coefficient were reported for the optical properties of the graphene nanofluids. An enhancement in the heat collection efficiency was found to increase with graphene addition into the pure oil. In a comparative study between graphene/oil nanofluid and CuO/oil nanofluid, the former displayed a superior heat transfer collection when applied to DASC. This could be attributed to the superior thermal conductivity of graphene nanofluid. Rose et al. (2017) developed a computational wave optics model with the aid of COMSOL to numerically assess the absorption

properties of graphite oxide (GO) nanofluid. The outcome was compared to the experimental result obtained using reflectance and transmittance spectrometry. The numerical results mostly agreed with the experimental results and only difference of 1 absorbance unit was reported for the absorption of some fluids, which include nanofluids with volume fraction of 0.008%, 0.012% and 0.014%. With experimentation, the optical characteristics of nanofluids of EG and GO of different fractions (0.004e0.016 vol%) were evaluated. The outcome revealed that an optimum volume concentration of 0.012 vol% GO is enough to produce a minimum reflectance and significantly high absorption for a solar receiver over the visible spectral range (380e800 nm). Shende and Ramaprabhu (2017) studied the enhancement in the efficiency of DASC using nanofluids as the absorbing medium. The optical and thermal properties of reduced graphene oxide (RGO) based nanofluids were investigated. The nanofluids were prepared by suspending specific concentrations (0.005, 0.01 and 0.03 vol%) of functionalized RGO in deionized water and EG. The absorption performance of the nanofluid was found to be better than that of the base fluids while the nanofluid extinction coefficient displayed remarkable improvement even at low nanofluid concentration of 0.005 vol%. To improve the photothermal efficiency of a GO/water nanofluid and Graphene/water nanofluid, Chen et al. (2017) prepared an RGO nanofluid by the irradiation of GO/water at different times (260e380 s) under ultra-violet light. The nanofluid was found to exhibit a superior photothermal conversion efficiency of about 96.63% at 30  C (303 K) and 52% at 75  C (348 K) in comparison to GO/water and graphene/water nanofluid of the same volume fraction. This demonstrated RGO nanofluid exhibit higher absorption property and enhanced photothermal performance, thus making it a promising absorbing medium for low-temperature DASCs. Recently, Xu et al. (2019) assessed the performance of a DASC using RGO/water-EG nanofluid with antifreeze property. The stability of the nanofluid and the antifreeze property were evaluated. The nanofluids sample were prepared by dispersing 0.01e0.10 wt% of GO in a 50:50 blend of water and EG with 0.015 wt% PVP as dispersant. However, only 0.06 wt% nanofluid was used for preparation of RGO nanofluid due to its higher zeta potential value, which indicates the best stability. The RGO nanofluid was prepared through irradiation using an ultraviolet lamp for 240s. Higher zeta potential of the RGO/water-EG nanofluid was reported at a higher temperature, which indicates good suspension stability at increased temperature. Also, the water-EG based nanofluids were found to be applicable in cold weather of up to about 47  C (226 K), which suggests a superior antifreeze performance to

A. Borode et al. / Journal of Cleaner Production 241 (2019) 118311

water-based nanofluids. Further observation shows that the waterEG based nanofluid exhibits better optical absorption and photothermal conversion property when compared to the base fluid and water-based nanofluid. The receiver efficiency of DASC using RGO/ water-EG based nanofluid was found to increase by 70% in comparison to the base fluid on exposure to solar intensity of 1000 W/ m2 for 60000s, while the temperature also increases by 76.9%. This shows RGO/Water-EG nanofluid has a huge potential to replace base fluid in a DASC. A numerical study using MATLAB was also used to validate the experimental result. The outcome displayed good agreement with the experimental result. Beicker et al. (2018) investigated the photothermal properties of different concentrations of Gold/Water nanofluids and MWCNT/ water nanofluids. The experimental result demonstrated that the tested samples exhibit exceptional photothermal conversion ability even at low concentrations. The optimum nanofluid concentration was reported to be 0.002 vol% and 0.001% for gold nanofluid and MWCNT nanofluid. This shows that the lower concentration of MWCNTs is enough to enhance the photothermal conversion ability of a fluid in comparison to gold-based nanofluid. This could be ascribed to the remarkable thermal conductivity of MWCNTs. Khosrojerdi et al. (2017) studied the thermo-optical properties of graphene oxide nanoplatelets (GONP) based nanofluids used as the absorbing medium in a low-temperature DASC. The examined nanofluid samples were prepared by dispersing different weight concentrations (0.001, 0.005, 0.015 and 0.045%) of GONP into deionized water. As presented in Fig. 17, experimental results revealed that the examined nanofluids exhibit a superior capacity to absorb solar energy than the base fluid within the wavelength range of 200e250 nm. Also, as the concentration increases, the ability of the nanofluid to absorb solar energy and the extinction coefficient increases. To absorb the solar energy fully, the authors recommended a minimum height of 0.03 m of the nanofluid layer for the nanofluid with a weight concentration of 0.045%. Moreover, the ability of the nanofluid to absorb energy is 99.6%. Recently, Choi et al. (2018) assessed the influence of different surfactants (SDBS, CTAB, SDS and Triton X-100) on the solar absorption properties of MWCNT nanofluids employed in a volumetric solar absorption receiver. They evaluated the influence of temperature on the stability of the surfactants based nanofluids. With the aid of an ultravioletevisible spectrophotometer, the spectral extinction coefficient was determined in a wavelength

15

range of 200e1800 nm after 3 h of production and 30 days. They observed a higher extinction coefficient for the MWCNT nanofluids than the base fluid. The extinction coefficient was found to be higher at increased MWCNT loading. Moreover, the experimental outcomes clearly revealed an increase in solar energy absorption. As obtained in Table 5, the surfactants also enhanced the solar absorption properties in the following decreasing order Triton X100 > SDBS > CTAB > SDS. The reduction in absorption rate after 30 days also follows the same trend. The surfactant-enhanced absorption order correlates with the degree of nanofluid stability achieved with the various surfactants. This proves that the solar absorption properties of the nanofluid are subjected to its stability. Further, SDBS was recommended as the most applicable of all the surfactants owing to its very good stability and absorption properties at elevated temperature (383e358 K). Triton X-100, CTAB and SDS were found not suitable as dispersants for preparing nanofluids to be used at elevated temperature due to the agglomeration of particles at an operating temperature between 383 and 358 K. This could be attributed to the failure of the surfactants at elevated temperature. However, detailed studies are still required to verify the behaviour of such surfactants at elevated temperature. In conclusion, carbon-based nanofluids have a better absorption property than base fluids. Thus, it is a better absorbing medium in a DASC. The performance of DASC with carbon-based nanofluid as an absorbing medium is summarized in Table 6.

3.4. Parabolic trough solar collector (PTSC) PTSC is a polished metal mirror collector that is curved like a parabola on one side and straight on the other side. Fig. 18 displays a representation of a typical PTSC. This collector focuses the

Table 5 Solar Absorption rate for MWCNT nanofluid with various surfactants (Choi et al., 2018). Period after Preparation

Surfactants Triton X-100

SDBS

CTAB

SDS

3h 30 days Reduction rate

98.84% 98.73% 0.111%

98.64% 98.36% 0.283%

97.69% 96.82% 0.890%

97.33% 94.26% 3.154%

Fig. 17. The absorption of GONP nanofluids at different wavelengths (Khosrojerdi et al., 2017).

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Table 6 Summary of findings on DASC using Carbon-based nanofluids. Author

Nanomaterial

Base Fluid

Concentration Findings

Otanicar et al. (2010) Ladjevardi et al. (2013) Luo et al. (2014)

CNT, Graphite, Silver

Water

0e1 vol%

Graphite

Water

0.000025 vol%  50% absorption of solar irradiation in comparison to that of water, which is 27%

Graphite, Long & Short CNT, Ag, Al2O3, SiO2, Cu

Water

0.01 vol%

Water, EG, PG, Therminol VP-1 [HMIM]BF4

0e53 mg/L

Hordy et al. (2014) Plasma f-MWCNT Liu et al. (2015)

Graphene

Zhang et al. (2016) GNP SWCNT Graphene Delfani et al. MWCNT (2016) Vakili et al. GNP (2016a)

(BMIM)BF4

0.0005 e0.001 wt% 0.0005 e0.01 wt%

Water þ EG 25e100 ppm (70:30) De-ionized Water 0.00025 e0.005%

 5% improvement in collector efficiency

   

 70% receiver efficiency with collector height of 0.05 m and 0.0005 wt%  Photothermal efficiencies between 21.3 and 26.6% in contrast to that of base fluid, which is 18.2%         

Gorji and Ranjbar Graphite, Magnetite, Silver Water (2016)

5e40 ppm

Rose et al. (2017) GO

EG

Wang et al. (2017) GNP

Base oil

f-rGO Shende and Ramaprabhu (2017) Chen et al. (2017) RGO

DIW

0.04e0.016 vol% 0.02e1.0 mg/  mL  5e75 ppm  

Khosrojerdi et al. (2017) Mehrali et al. (2018) Beicker et al. (2018) Choi et al. (2018)

GONP

Water

Hybrid Ag-RGO

Water

MWCNT

Water

MWCNT

Water

RGO

Water

Xu et al. (2019)

Water

Increase in outlet temperature by 30e100 K Improvement in collector efficiency by 2e25% Photothermal efficiency of 122.7% for 0.01% graphite nanofluid Achieved 100% solar energy absorption

Increase in collector efficiency with increase in concentration and flow rate Efficiency augmentation of 10e29% more than that of the base fluid Complete solar absorption at a wavelength of 286 nm Increase in GNP loading enhances absorbance coefficient Reduction in collector height Thermal efficiency of 33e57% Exergy efficiency of 13e20% Outlet temperature is elevated with an increase in nanofluid concentration An optimum concentration of 0.012 vol% produces the minimum reflectance and very high solar absorption Enhancement in collector efficiency with the addition of graphene High absorption coefficient, high extinction co-efficient and low scattering coefficient Improvement in extinction coefficient Increase in absorption with increase in concentration

 Photothermal conversion efficiency of up to 96.63% at 30  C (303 K) and 52% at 75  C (348 K) greater than GO/Water and Graphene/Water nanofluids 0.001  Maximum solar absorption of 99.6% for nanofluid with the highest concentration of e0.045 wt% 0.045 wt% 10e100 ppm  Collector efficiency of 77% at a nanofluid concentration of 40 ppm and collector height of 0.02 m 0.001e0.004%  Enhanced photothermal conversion ability  Optimum concentration is 0.001% 0.0005e0.002  Extinction coefficient and absorbance rate of the nanofluid much higher than that of water  Enhancement in solar absorption rate with the addition of different surfactant in decreasing order Triton X-100 > SDBS > CTAB > SDS 0.06 wt%  Temperature elevation of 76.9% and efficiency increase of 70% for exposure to the solar intensity of 1000 W/m2 for 6000 s 0.02 wt%

Fig. 18. Representation of a typical PTSC (Bellos and Tzivanidis, 2019).

incident sunlight to a focal line with a tube receiver. Thereafter, the working fluid flowing through the tube is heated up to a very high temperature. The hot fluid could be used in heat turbines or steam engines for power generation. PTSC is mainly applied in concentrated solar power plants and other processes that require a temperature range of 373e523 K (Kalogirou and Kalogirou, 2009b;

Pitz-Paal, 2014). The efficiency of this collector could be improved by adding nanomaterials. However, limited research studies exist on PTSC using carbon-based nanofluid. In a bid to enhance the thermal performance of a solar parabolic trough collector, Kasaeian et al. (2015) used a nanofluid prepared by suspending 0.1 wt% and 0.3 wt% of CNT in mineral oil. The outcomes revealed a global efficiency enhancement of 4e5% and 5e7% for 0.1 wt% and 0.3 wt% MWCNT nanofluids respectively in comparison to the pure conventional oil. This revealed the huge potential of using nanofluids to remarkably improve the performance of the solar collector. Kasaeian et al. (2017) carried out a comparative study of the performance of a parabolic trough collector utilizing two different nanofluids. Three different receivers were used in the collector and they include a bare glass tube, non-evacuated glass tube and a vacuum absorber tube. The nanofluids were prepared by suspending 0.2 and 0.3 wt% of silica and CNTs in EG. The thermal efficiency and the outlet temperature of the fluids were evaluated. As shown in Table 7, for the PTSC with all the receivers, there was an enhancement in the thermal efficiency and temperature difference as the concentration of both silica and CNT nanofluid was increased from 0.1 wt% to 0.3 wt%. However, using MWCNT nanofluid in the collector produced better thermal performance. The study revealed that the MWCNT nanofluid has the highest thermal efficiency of

A. Borode et al. / Journal of Cleaner Production 241 (2019) 118311

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Table 7 Result showing the fluid outlet temperature and thermal efficiency of parabolic trough collector with different receivers (data extracted from literature) (Kasaeian et al., 2015). Nanofluids

Concentration (vol%) Bare Glass Tube

Non-Evacuated Glass Tube

Vacuum Absorber Tube

Outlet temp. (K) Thermal Efficiency (%) Outlet temp. (K) Thermal Efficiency (%) Outlet temp. (K) Thermal Efficiency (%) e 0.2 0.3 MWCNT Nanofluid 0.2 0.3

Water Silica Nanofluid

312.9 317.1 318.7 322.6 327.2

52.2 55.9 57.3 61 65.6

74.9% and fluid outlet temperature of 338.3 K in the vacuum absorber tube. The thermal efficiency for the vacuum absorber tube was found to be about 20% more than that of the bare glass tube on the average. Further studies reported an enhancement of 70.9% and 80.7% for the collector with vacuum absorber tube using nanofluids with optimum volume fraction 0.4 vol% silica and 0.5% MWCNTs respectively. Mwesigye et al. (2018) performed a numerical investigation to evaluate the performance of a parabolic trough solar collector utilizing SWCNT-Therminol VP-1 based nanofluid as the working fluid. Despite having a significant enhancement of about 234% in thermal conductivity, the thermal efficiency was found to increase insignificantly by about 4.4% as the volume concentration of the nanofluid is increases from 0 to 2.5 vol%. This was accompanied by a corresponding cutback of about 70% in the entropy generation. The outcome obtained from the studies in this section revealed that carbon-based nanofluids were able to improve the thermal efficiency of a PTSC from about 4% to 80.7%. This is ascribed to the significant intensification in the thermal conductivity of the nanofluid. However, most of the studies focused on the use of MWCNT nanofluids while little to no literature exists on the use of graphene nanofluid in a PTSC. Table 8 summarizes the major findings on PTSC utilizing carbon-based nanofluids.

3.5. Hybrid photovoltaic thermal (PVT) collector A hybrid PVT collector combines a PV module and a thermal collector to generate power and thermal energy by converting solar irradiation with a single system (Tripanagnostopoulos, 2012; Tyagi et al., 2012). One of the major reasons for the lower efficiency of a PV panel is the higher temperature of the solar cells at higher irradiation (Dubey et al., 2013; Thong et al., 2016). Thus, a hybrid PVT collector allows for more efficient conversion of incident solar radiation to electricity than to a PV system. This is due to a reduction in the PV module or solar cell temperature through heat transport with the fluid in the thermal collector for other heating application (Joshi and Dhoble, 2018). This hybrid system is advantageous for space saving while taking advantage of solar energy for electricity production and heating purpose. Fig. 19 is a schematic of a typical hybrid PVT collector. However, there are minimal studies

317.2 321.6 325.2 325.9 331.3

56.1 59.6 63.3 64.2 71.9

321 325.7 327.7 329.3 333.8

59.5 64.1 66.5 68.5 74.9

Fig. 19. Schematics of a typical hybrid PVT Collector (Khelifa et al., 2016).

on the application of carbon-based nanofluids in a hybrid PVT system. Nasrin et al. (2018) conducted experimentation and a numerical study to evaluate the actual performance of a PVT system using MWCNT nanofluid as thermal fluid. As depicted in Fig. 20, the authors designed and developed a new PVT collector with an effective pipe arrangement to achieve better heat transfer. A 3D numerical study was performed with COMSOL software to evaluate the performance of the system with water and MWCNT nanofluid. The results were validated with indoor experimentation at constant mass flow rates of 0.5 L/min and MWCNT loading in water ranging from 0 to 1 wt% under irradiation level range from 200 to 1000 W/ m2 and inlet fluid temperature of 32  C (305 K). Fig. 21 depicts the influence of solar irradiation and nanofluid concentration on electrical and thermal power and efficiency. The maximum temperature of the PVT panel was found to increase from 56.4 to 84.3  C (329.4e357.3 K) with an increase in irradiation level from 200 to 1000 W/m2. This equates to a surface temperature increase of 3.49  C (276.49 K) per 100 W/m2 solar radiation. The study further revealed that despite realizing an increase in electrical power output and thermal energy with increase in irradiation, there was a decline in electrical efficiency and thermal efficiency. This signifies that heat transfer from the system to the working fluid is not

Table 8 Summary of findings on PTSC using Carbon-based nanofluids. Author

Nanomaterial Base Fluid Concentration Findings

Kasaeian et al. Silica (2015) CNT

EG

0.2 vol% 0.3 vol%

Kasaeian et al. CNT (2017) Mwesigye SWCNT et al. (2018)

Mineral Oil 0.1e0.3 vol% Therminol 2.5 vol% VP-1

 CNT nanofluid produced the highest temperature of 338.3 K and collector efficiency of 74.9% in the vacuumed glass-glass absorber tube  4e5% and 5e7% enhancement in the efficiency of a collector using 0.1 and 0.3 wt% nanofluid respectively when compared to water 80.7% efficiency for optimum CNT loading of 0.5 vol% and 70.9% for 0.40% silica loading  4e5% and 5e7% enhancement in the efficiency of a collector using 0.1 and 0.3 wt% nanofluid respectively when compared to water  4% enhancement in efficiency with increase in SWCNT loading from 0 to 2.5 vol%  70% drop in entropy generation

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Fig. 20. Schematics of the developed PVT (Nasrin et al., 2018).

Fig. 21. Effect of Solar irradiation on the (a) Electrical Power (b) Electrical Efficiency (c) Thermal Energy (d) Thermal Efficiency (Nasrin et al., 2018).

effective and more heat is lost to the environment at higher solar radiance. Notwithstanding, the nanofluid outlet temperature was found to increase with an increase in solar radiance and MWCNT loading. Moreover, the report presented details of the effect of nanofluid concentration on the PVT system performance at the solar radiance of 1000 W/m2 as presented in Fig. 22. A decrease of about 2.1  C (275.1 K) was realized in the solar cell temperature for the MWCNT loading increase from 0 to 1 wt%. This proves that the nanofluid is a

more effective cooling fluid than water due to their significantly higher thermal conductivity. However, a reduction in the average solar cell temperature was observed with 1 wt% nanofluid coupled with an increase in pumping power and an increased tendency for the agglomeration of nanomaterials. This indicates that optimal nanofluid concentration for the best PVT performance is lesser than 1.0 wt%. In the experimental study, the power output of the PVT collector was observed to increase from 178.73 W to 180.8 W when the MWCNTs concentration in water was increased from 0 to 1 wt%

A. Borode et al. / Journal of Cleaner Production 241 (2019) 118311

19

Fig. 22. Effect of weight fraction (a) Electrical Power (b) Electrical Efficiency (c) Thermal Energy (d) Thermal Efficiency at solar irradiation of 1000 W/m2 (Nasrin et al., 2018).

at the solar radiance of 1000 W/m2 while the thermal energy increases from 1088.9 W to 1144.5 W. In the same order, electrical efficiency increases from 11.82 to 11.96%. However, in the numerical study, the thermal energy and electrical efficiency increase from 1105.9 to 1165.1 W and 11.95e12.1% respectively. This was ascribed to improved convective HTC at higher nanofluid concentration and lower solar cell temperature. In summary, when nanofluid was used in a PVT system in comparison to water, a thermal efficiency increase of 4% and 3.67% with electrical performance augmentation of 0.15% and 0.14% was obtained in the numerical and experimental study respectively. Meanwhile, with an increase in irradiation from 200 to 1000 W/m2, the overall efficiency was found to decrease from 92.5 to 95% to 87.65 and 89.2% in the numerical and experimental study respectively. However, the overall efficiency is higher at higher weight fraction. Fayaz et al. (2018) assessed the effect of flow rate on the efficiency of a PVT system operating with MWCNT nanofluid. The nanofluid concentration was varied between 0 and 1.0 wt%. The preliminary numerical study revealed that the thermal efficiency of the PVT increases with increase in nanofluid concentration up to 0.75 wt%, after which the thermal efficiency starts reducing. This indicates that 0.75 wt% is the optimum weight concentration. Subsequently, a 3D numerical analysis was done using COMSOL software and validated with indoor experimentation for the 0.75 wt% MWCNT nanofluid at inlet temperature of 32  C (305 K) and flow rates range of 0.5e2 L/min with solar radiance kept at 1000 W/m2 and surrounding temperature of 25  C (298 K). The research outcomes are presented in Fig. 23. They realized a reduction in cell temperature at higher flow rate coupled with an increase in output power and electrical power. This proves that a reduction in solar cell temperature improves the electrical power and efficiency. They reported an increase of 0.95 and 1.01 W in output power and 0.064 and 0.067% electrical efficiency for every

1  C (274 K) reduction in the cell temperature in the experimental and numerical study respectively. In addition, the thermal energy and efficiency were improved with an increase in flow rate while the outlet nanofluid temperature reduces. In the experimentation, they noted a higher electrical efficiency, thermal efficiency and overall efficiency increment of 11.02%, 79.1% and 5.73% respectively for the PVT system at the highest flow rate of 2 L/min. Numerically, they realized a higher electrical efficiency, thermal efficiency and overall efficiency increment of 11.29%, 81.24% and 6.26% respectively under the same operating parameters. However, assessing the performance of the nanofluid-based PVT system with respect to the flow rate tends to overestimate the impact of the nanofluid. This is because there will be an improvement in the efficiency of any heat exchanging device or solar collectors when the flow rates are increased regardless of the working fluid utilized (DembeckKerekes et al., 2019; Fayaz et al., 2019; Fudholi et al., 2014; Pal Singh et al., 2019). Future studies should focus more on the corrosive and erosive effect of nanofluids and the effect of flow rates on the corrosion-erosion rate. Abdallah et al. (2019) performed outdoor experimentation to analyse the performance of a hybrid PVT system with MWCNT nanofluid as the heat absorption fluid. The impact of nanofluid concentration between 0 and 0.3 vol% was evaluated at a flow rate of 1.2 L/min. As presented in Fig. 24, the temperature of the PV modules was higher at mid-day. This correlates with when the highest solar radiation is obtained. All through the day, the noncooled PV was observed to have a higher temperature than the cooled PV. Moreover, the nanofluid-cooled PV is cooler than watercooled PV with the lowest temperature achieved while using MWCNT nanofluid with optimal concentration of 0.075 vol%. A temperature diminution of 12  C (285 K) was reported for 0.075 vol % MWCNT nanofluid at peak hour and an average reduction of 10.3  C (283.3 K) all day. However, the increased PV modules temperature during the peak hour result in a reduction in electrical

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Fig. 23. Influence of mass flow rate on (a) Output power (b) Electrical Efficiency (c) Thermal Energy (d) Thermal Efficiency (Fayaz et al., 2018).

efficiency as presented in Fig. 25. Notwithstanding, the overall efficiency is highest during this peak hour. Moreover, electrical efficiency enhancement of 26.4% was reported for 0.075 vol% MWCNT nanofluid-cooled PV module in contrast to the non-cooled PV module. The average overall efficiency for the day is 61.23% with a reported maximum overall efficiency of 83.62% at peak hour. From the studies in this section, it can be concluded that while the PV module or solar cell temperature is increased with increase in incident solar radiation, the temperature of the system is lower and the overall efficiency is higher with the application of nanofluid in comparison to water or no-cooler. Owing to the addition of nanomaterials, this performance improvement could be ascribed to the intensification in the Brownian motion (Jabbari et al., 2017; Shukla et al., 2016) or clustering (Daviran et al., 2017) that consequently raises thermal conductivity and HTC. More studies need to be done on other carbon-based nanofluids. Moreover, the existing reports didn't address the long-term stability of the nanofluids and the stability at elevated temperature. Also, the tendency of the nanomaterials to adhere to the tube wall is a cause for concern and could result in a reduction of the nanomaterial concentration in the working fluid and subsequently lead to blockage of the channel. Summary of the performance of PVT systems working with carbonbased nanofluids is presented in Table 9. 4. Conclusion This paper presents an overview and summary of the utilization of carbon-based nanofluids in different solar collectors. This includes flat plate solar collector, evacuated tube solar collector, direct absorption solar collector, parabolic trough solar collector and hybrid PVT collector. The application of carbon-based nanofluids in solar collectors produces an improvement in the thermal efficiency of the collectors. Thus, carbon-based nanofluids have a huge potential to not only improve energy efficiency but to also reduce the size of collectors, consequently reducing manufacturing

cost. From the studies reviewed, the following conclusions could be drawn: i. The thermal conductivity of working fluid is improved with the incorporation of nanomaterials, consequently improving heat transfer rate and solar energy absorption. This improvement is ascribed to the Brownian motion intensification that causes turbulence effects due to the erratic movement of particles, thus enhancing heat transfer. ii. The viscosity of the working fluid is increased with the addition of nanomaterials. This creates a major challenge for the incorporation of nanofluid in solar collectors except for DASC. This is due to the consequent increase in pumping power. iii. The minimization of entropy generation was achieved with the utilization of carbon-based nanofluids. Thus, the energy output is maximized. iv. The efficiency of solar collectors using nanofluids is subjected to the concentration of the nanofluid. Mass flow rates and solar radiance. Finally, the following challenges and scope for future studies are identified: i. The unwanted increase in viscosity, which results in a higher pressure drop, needs to be addressed. ii. The is no standard for preparing nanofluid. No specific nanomaterials morphology, concentration or surfactants have been established to achieve the highest possible thermal conductivity, insignificant increase in viscosity and longterm stability of the nanofluid. iii. The expensive cost of nanomaterials is a huge hindrance to the commercial application of carbon-based nanofluids in a solar thermal system.

A. Borode et al. / Journal of Cleaner Production 241 (2019) 118311

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Fig. 24. (a) Solar radiation and (b) Temperature of the PV module at different daylight hour of a day (Abdallah et al., 2019).

iv. The long-term stability of nanofluid is also essential for its commercialization. Nanomaterials are known to adhere to the inner tube walls. This adherence coupled with the aggregation of nanomaterials could lead to clogging of channels and reduction in nanomaterials concentration. v. There are contradicting results as to the suppressing performance of SDS on the thermal performance of carbonbased nanofluid. This should be clarified with a detailed experimental and numerical study using molecular dynamic simulation. vi. A detailed study is required to determine the performance of carbon-based nanofluids in comparison to other nanofluids such as CuO based nanofluid. This is due to the contradicting reports that exist as regards the superior thermal performance of MWCNT nanofluid when compared to CuO nanofluid vii. Also, since surfactants are used to improve stability, it is quite important to evaluate its long-term performance and its behaviour at an elevated temperature more than 70  C viii. A detailed study should be conducted to evaluate the effect of surfactant additives on the efficiency of solar collectors.

ix. Studies on the application of carbon-based nanofluids in a hybrid photovoltaic thermal system are quite limited. More studies should be conducted to determine the improvement in the thermal and electrical efficiency of the system. x. More research is needed to assess the techno-economic impact and environmental benefits of carbon-based nanofluid in achieving cleaner energy production through enhanced energy efficiency. This should include a detailed report of CO2 and SO2 emission reduction achieved by replacing coal usage with nanofluid-based solar collectors and other heat transfer devices. xi. An in-depth understanding of the heat transport mechanism responsible for thermal conductivity intensification is still required. xii. It is important to verify the effect of low temperature obtainable in ice-cold weather region on the stability and thermophysical properties of carbon-based nanofluids with antifreeze. xiii. Finally, the corrosive and erosive impact of carbon-based nanofluids should be determined. This should also include the effects of surfactants and flow rates.

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Fig. 25. (a) Electrical Efficiency and (b) Overall efficiency of the PVT collector at different daylight hours of a day (Abdallah et al., 2019).

Table 9 Summary of the performance of a hybrid PVT system working with carbon-based nanofluids. Ref.

Nasrin et al. (2018) Fayaz et al. (2018) Abdallah et al. (2019)

Working fluid

Water þ1.0 wt% MWCNT Water þ0.75 wt% MWCNT Water þ0.05 wt% MWCNT þ0.075 wt% MWCNT þ0.1 wt% MWCNT þ0.2 wt% MWCNT þ0.3 wt% MWCNT

PVT Specification

Efficiency

Material

Size (mm)

Power (W)

Electrical

Thermal

Overall

Polycrystalline

1984  997

295

Polycrystalline

1984  997

295

Monocrystalline

250  345

10

11.82% 11.96% 12.3% 12.37% 16.4% 23.4% 33.9% 30.9% 24.2% 20.13%

72.02% 75.69% 75.24% 79.1% N/A

83.84% 87.65% 86.51% 91.47% 40.9% 66.8% 83.26% 76.08% 70.7% 97.3%

A. Borode et al. / Journal of Cleaner Production 241 (2019) 118311

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